ZIF-8 crystal coatings on a polyimide substrate and their catalytic behaviours for the Knoevenagel reaction.

ZIF-8 crystal coatings were prepared by direct growth on different flexible polyimide substrates including a thin membrane and electrospun nanofiber mat. In SEM pictures, the ZIF-8 crystals exhibit a rhombic dodecahedron morphology. Owing to the flexible polyimide substrate, the MOF-polymer film may be moderately bent or easily tailored when not destroyed. The ZIF-8 coatings can be activated at high temperature owing to the excellent thermal resistance and low coefficient of thermal expansion of the polyimide. Furthermore, this material may be used as an efficient heterogeneous catalyst for the Knoevenagel reaction and can be easily recovered. The ZIF-8 coatings and 2-methylimidazole show similar catalytic behaviour as a weak base.